5-aminolevulinic acid (ALA) is an endogenous precursor for photosensitizers formed by heme biosynthesis, and has been studied for photodynamic therapy (PDT) with promising results for some skin diseases.
When absorbed into the skin through topical application, such as coating or patching, ALA migrates into cells and is converted into protoporphyrin IX (PpIX), which is used as an endogenous photosensitizer in photodynamic therapy (PDT).
ALA, under the same conditions, is more rapidly absorbed into abnormal cells or tissues than normal cells or tissues.
PpIX is induced to be fluorescent when exposed to visible light in the frequency range from 450 to 650 nm. Transferring energy to adjacent oxygen molecules, this fluorescence converts them into singlet oxygen, a reactive oxygen species, which functions as a toxin destroying the adjacent cells and tissues thereof.
Therefore, radiation leads to various therapeutic effects on abnormal cells and tissue (skin) in which a high concentration of PpIX is accumulated.
However, the high hydrophilicity and low hydrophobicity thereof prevents ALA from penetrating into such barriers as the stratum corneum and cell membranes. It takes a very long time to accumulate ALA in cells at a sufficient concentration to elicit the desired therapeutic effects.
To address this limitation of uptake and distribution of ALA, the drug has been converted into its esters, such as methyl ester, butyl ester, etc. These alkyl esters are greatly decreased in hydrophilicity compared to carboxylic acid, which makes it easy for them to pass through barriers such as the stratum cornea and cell membranes. However, the compounds find applications only in narrow and limited fields.
Moreover, since the synthesis of the ester derivatives involves a complex process including the introduction of an amino-protecting group and the removal thereof, the production yield becomes too low to apply the process in practice.
Thus, there is a need for a method by which ALA can be effectively carried into cells and which can find application in a variety of fields.
Leading to the present invention, intensive and thorough research into the effective delivery of ALA into cells, conducted by the present inventors, resulted in the finding that unsaturated alkyl esters of ALA can readily penetrate into barriers such as the stratum cornea and cell membranes. Nowhere are such unsaturated alkyl esters of ALA disclosed in the prior art.
Also, a method for synthesizing the esters of ALA at a high yield without using amino-protecting groups was also developed, which can be applied to a variety of fields.